"Start Speaking" command and Voice Over always uses Alex voice

I'd like to set up my Mac so it can read aloud text on news websites, but I don't like the default system voice called "Alex". I prefer the "Siri" voices that you can download and enable in System Preferences.

However, no matter which voice I enable in System Preferences > Accessibility > Spoken Content, my Mac always uses the Alex voice for everything, even though I have (ostensibly) disabled or "deleted" the Alex voice, along with the other voices that come enables by default.

I would like to use the command Speech > Start Speaking and Stop Speaking commands in Safari to read text on websites that I view using Reader mode.

Thanks for your help.